Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 2 Apr 2018 16:57:03 -0700 (PDT)
From:   David Rientjes <rientjes@google.com>
To:     Laurent Dufour <ldufour@linux.vnet.ibm.com>
cc:     paulmck@linux.vnet.ibm.com, peterz@infradead.org,
        akpm@linux-foundation.org, kirill@shutemov.name,
        ak@linux.intel.com, mhocko@kernel.org, dave@stgolabs.net,
        jack@suse.cz, Matthew Wilcox <willy@infradead.org>,
        benh@kernel.crashing.org, mpe@ellerman.id.au, paulus@samba.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, hpa@zytor.com,
        Will Deacon <will.deacon@arm.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        kemi.wang@intel.com, sergey.senozhatsky.work@gmail.com,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        haren@linux.vnet.ibm.com, khandual@linux.vnet.ibm.com,
        npiggin@gmail.com, bsingharora@gmail.com,
        Tim Chen <tim.c.chen@linux.intel.com>,
        linuxppc-dev@lists.ozlabs.org, x86@kernel.org
Subject: Re: [PATCH v9 16/24] mm: Introduce __page_add_new_anon_rmap()
In-Reply-To: <1520963994-28477-17-git-send-email-ldufour@linux.vnet.ibm.com>
Message-ID: <alpine.DEB.2.20.1804021655100.253461@chino.kir.corp.google.com>
References: <1520963994-28477-1-git-send-email-ldufour@linux.vnet.ibm.com> <1520963994-28477-17-git-send-email-ldufour@linux.vnet.ibm.com>
User-Agent: Alpine 2.20 (DEB 67 2015-01-07)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, 13 Mar 2018, Laurent Dufour wrote:

> When dealing with speculative page fault handler, we may race with VMA
> being split or merged. In this case the vma->vm_start and vm->vm_end
> fields may not match the address the page fault is occurring.
> 
> This can only happens when the VMA is split but in that case, the
> anon_vma pointer of the new VMA will be the same as the original one,
> because in __split_vma the new->anon_vma is set to src->anon_vma when
> *new = *vma.
> 
> So even if the VMA boundaries are not correct, the anon_vma pointer is
> still valid.
> 
> If the VMA has been merged, then the VMA in which it has been merged
> must have the same anon_vma pointer otherwise the merge can't be done.
> 
> So in all the case we know that the anon_vma is valid, since we have
> checked before starting the speculative page fault that the anon_vma
> pointer is valid for this VMA and since there is an anon_vma this
> means that at one time a page has been backed and that before the VMA
> is cleaned, the page table lock would have to be grab to clean the
> PTE, and the anon_vma field is checked once the PTE is locked.
> 
> This patch introduce a new __page_add_new_anon_rmap() service which
> doesn't check for the VMA boundaries, and create a new inline one
> which do the check.
> 
> When called from a page fault handler, if this is not a speculative one,
> there is a guarantee that vm_start and vm_end match the faulting address,
> so this check is useless. In the context of the speculative page fault
> handler, this check may be wrong but anon_vma is still valid as explained
> above.
> 
> Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>

I'm indifferent on this: it could be argued both sides that the new 
function and its variant for a simple VM_BUG_ON() isn't worth it and it 
would should rather be done in the callers of page_add_new_anon_rmap().  
It feels like it would be better left to the caller and add a comment to 
page_add_anon_rmap() itself in mm/rmap.c.